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Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters

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Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters Review Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters Yi Yang and Joseph J. Pignatello * Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, 123 Huntington St., P.O. Box 1106, New Haven, CT 06504-1106, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-203-974-8518 Received: 18 September 2017; Accepted: 4 October 2017; Published: 13 October 2017 Abstract: Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes and in reactions taking place during photochemical water treatment. While inert to solar wavelengths, halides can be converted into radical and non-radical reactive halogen species (RHS) by sensitized photolysis and by reactions with secondary reactive oxygen species (ROS) produced through sunlight-initiated reactions in water and atmospheric aerosols, such as hydroxyl radical, ozone, and nitrate radical. In photochemical advanced oxidation processes for water treatment, RHS can be generated by UV photolysis and by reactions of halides with hydroxyl radicals, sulfate radicals, ozone, and other ROS. RHS are reactive toward organic compounds, and some reactions lead to incorporation of halogen into byproducts. Recent studies indicate that halides, or the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of dissolved natural organic matter (DOM); alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, DOM, and pollutants; and give rise to certain halogen oxides of concern as water contaminants. The complex and colorful chemistry of halogen in waters will be summarized in detail and the implications of this chemistry for global biogeochemical cycling of halogen, contaminant fate in natural waters, and water purification technologies will be discussed. Keywords: hydroxyl radical; sulfate radical; photocatalysis; atmospheric aerosols; reactive oxygen species; reactive halogen species; advanced oxidation processes; dissolved natural organic matter; halogenation; reclaimed waters 1. Introduction Halide ions are ubiquitous in natural waters. Ordinary levels of halides in seawater are 540 mM chloride, 0.8 mM bromide, and 100–200 nM iodide [1,2]. Halide levels range downward in estuaries and upward in saltier water bodies relative to typical seawater levels. Surface fresh water and groundwater may contain up to 21 mM chloride and 0.05 mM bromide [1], with higher levels in some places. Even though the halides themselves do not absorb light in the solar region, in nature they provide far more than just background electrolytes—they participate in a rich, aqueous-phase chemistry initiated by sunlight that has many implications for dissolved natural organic matter (DOM) processing, fate and toxicity of organic pollutants, and global biogeochemical cycling of the halogens. Advanced oxidation processes (AOPs) employing solar, visible, or ultraviolet light have been used or are under study for removal of organic pollutants from reclaimable waters, such as industrial wastewater, petrochemical produced waters, municipal wastewater, and landfill leachates, in order to meet agricultural, residential, business, industrial, or drinking water standards. While generalizations are difficult, such waters often contain moderate-to-very-high halide ion concentrations, as well as high Molecules 2017, 22, 1684; doi:10.3390/molecules22101684 www.mdpi.com/journal/molecules Molecules 2017, 22, 1684 2 of 23 concentrations of other photochemically important solutes like carbonate that can impact halogen chemistry [1]. This review aims to summarize the reactions of halides and their daughter products and offer insight into their effects on photochemical transformations taking place in water. Halides can undergo sensitized photolysis and react with many secondary photoproducts to produce reactive halogen species (RHS) that can participate in a variety of reactions with DOM and anthropogenic compounds, including oxidation and incorporation of halogen. These reactions are described and discussed. Extensive tabulations of rate constants for relevant reactions or RHS generation and decay have been collected for the convenience of the reader in Supplementary Section Table S1. Halides, and the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of DOM; alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, dissolved natural organic matter, and pollutants; and give rise to certain halogen oxides of concern as water contaminants. The concentrations of halides is an important consideration in water treatment because halides can scavenge desired reactive oxidants and lead to unwanted halogenated byproducts. The identity of the halogen substituent(s) is critical because toxicity ordinarily increases in the order Cl < Br < I for compounds of similar structure [3,4]. Halogen reactions in the atmosphere have been well studied in relation to ozone chemistry [5]. This article will not discuss gas phase reactions or surface reactions in the atmosphere, a topic recently addressed in a comprehensive review [5]; however, it will cover relevant reactions that occur in the liquid phase or at the air-liquid interface of atmospheric aerosols. A number of important reactions that take place on snow, ice, and solid microparticles actually occur on or within a surface liquid layer that is often rich in salts [6]. Compared to bulk natural waters, aerosol liquid phases can reach lower pH, and the evidence supports altered rates and/or unique chemical reactions close to the air-liquid interface. 2. Sources and Speciation of RHS Produced from Halide Ions Reactive halogen species are generated by sensitized photochemical reactions or by reaction of halides with other oxidants of a photochemical origin. Halogen interconversion reactions are dealt with in detail. Scheme 1 provides an overview. Scheme 1. Generation of RHS in waters through the action of sunlight. 2.1. Sensitized Photolysis Halide ions in aqueous solution have absorption edges below ~260 nm and therefore do not photolyze at solar wavelengths. However, recent studies indicate that photo-sensitization by DOM Molecules 2017, 22, 1684 3 of 23 may be an important source of RHS in natural waters [7,8]. Irradiation of DOM with solar light generates a short-lived excited singlet state (1DOM*) that can relax to the ground state or intersystem crosses (ISC) to a much longer-lived excited triplet state (3DOM*). 3DOM* is a mixture of excited triplet states of diverse structures with energies ranging from 94 kJ·mol−1 to above 250 kJ·mol−1 [9]. While the nature of the chromophoric groups of DOM giving rise to triplet states is not known for certain, it has been said that aromatic ketones and other carbonyl-containing groups (e.g., coumarin and chromone moieties) are candidates for production of the high-energy triplet states of DOM [10]. The steady-state concentration of 3DOM* is estimated to be 10−14 to 10−12 M, depending on light intensities, [DOM] and [O2] [10] and, undoubtedly, the nature of DOM in the water parcel. 3DOM* is a known precursor of photochemically-produced reactive oxygen species (ROS) such as singlet oxygen (1O2) and hydroperoxyl/superoxide (HO2•/O2−•, pKa = 4.88), and is a suspected precursor of hydroxyl (HO•). In addition, 3DOM* also can engage in triplet energy transfer or oxidation reactions with itself and with other solutes. It has been shown that 3DOM* can oxidize or reduce various organic compounds [11], and that model triplet ketone sensitizers with similar reactivity as 3DOM* can oxidize CO32− to CO3−•, NO2− to NO2• [12], etc. The question arises whether 3DOM* can oxidize halide ions. The standard reduction potential of 3DOM* obtained in different studies of terrestrial and freshwater NOM reference standards is estimated to be “centered near 1.64 V” [10] and about 1.6–1.8 V [8]. The estimated one-electron E° reduction potentials of the halogens X/X - are 2.59 V (Cl), 2.04 V (Br), and 1.37 V (I) in water [13]. These values are about 0.4–0.5 V lower in polar organic solvents—an important consideration because DOM exists as supramolecular aggregates and colloids, in which the electric field in the vicinity of the chromophoric site may be somewhere in between water and polar organic solvents. It thus appears that bromide and iodide, and possibly chloride, are potentially susceptible to one-electron oxidation by 3DOM*. Jammoul et al. [7] found that the triplet excited state of benzophenone, which can be regarded as a surrogate for aromatic carbonyl compounds in seawater DOM, can oxidize halide ions to X2−•, Reaction (1): hv (355 nm) [(C6H5)2C=O]3* + 2X− ⎯⎯⎯⎯→ [(C6H5)2C-O]−• + X2−• (1) The rate constant for Reaction (1) follows the order, I− (~8 × 109) > Br− (~3 × 108) > Cl− (<1 × 106 M−1 s−1) which is consistent with the order in their reduction potential. The triplet state of anthraquinone derivatives was observed to oxidize bromide and chloride [12,14]. Building on previous theory [15], Loeff et al. [12] modeled reactions sensitized by simple organic compounds according to Scheme 2. X 3M + X 3 MX 3 MXX a b c d M + X M + X M + 2X M + X2 Scheme 2. Proposed pathways of
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